Japan booked their ticket to the knockout stage of the Asian Cup in the United Arab Emirates on Sunday with a 1-0 win over Group F opponents Oman, but the four-time champions were once again less than convincing.

Relying on a first-half penalty from Genki Haraguchi to give them victory, Hajime Moriyasu's side never looked settled despite creating several chances and dominating possession at Abu Dhabi's Zayed Sports City Stadium.

The result followed their 3-2 win over Turkmenistan on Wednesday -- another shaky display against an opponent they were expected to beat comfortably.

Moriyasu made two changes to the side that beat Turkmenistan, replacing striker Yuya Osako with Koya Kitagawa and bringing in defensive midfielder Wataru Endo for center-back Tomoaki Makino.

After netting a brace against Turkmenistan, Osako was rested to recover from a right gluteal strain that has been troubling him since before the start of the tournament.

Endo was cleared to play after missing the opening group clash due to a fever, allowing Takehiro Tomiyasu, who deputized as a holding midfielder against Turkmenistan, to return to his normal center-back role.

With Japan pinning Oman inside their own half early on, goalkeeper Faiyz Al Rusheidi kept his side in the contest with a valiant performance between the posts.

His five first-half saves included a pair in quick succession in the 24th and 26th minutes to twice deny Salzburg attacker Takumi Minamino, who looked set to score both times.

The first of the two saves saw the ball fall to 20-year-old striker Ritsu Doan, whose shot was blocked by the head of defender Khalid Al-Braiki.

Despite guessing the right direction, Al Rusheidi was unable to stop Haraguchi giving Japan the lead from the spot in the 28th minute after defender Ahmed Al Mahaijri was adjudged to have fouled the Hannover winger just inside the area. Replays, however, showed Haraguchi going to ground with minimal contact from Al Mahaijri, who appeared to make the initial challenge outside the box.

Oman had their own appeal for a penalty waved away in the final minute of first-half regulation after the ball appeared to take a deflection off the arm of Japan defender Yuto Nagatomo.

While Moriyasu praised his team for "playing hard," Haraguchi gave a severe assessment of their performance, saying major improvement was needed.

"It really wasn't good. We may have gotten the win and qualified for the next stage, but it was not a good performance," he said.

"We started with good pace in the first half and created chances early on, but the pace wasn't there in the second half. If we keep playing like this, we won't go far in the knockout stage."

Japan almost opened the scoring in the second minute when Haraguchi hit the crossbar from directly in front of goal after Doan beat a pair of defenders and pulled a cross back from the right of the box.

Minamino came within inches of an opener in the seventh minute after pouncing on a defensive mistake and making a run into the box, but he was denied one-on-one by Al Rusheidi.

Minamino then fired just wide to the right of the goal four minutes later when he ran onto a long ball over the top from Tomiyasu to get a clear shot from the left side of the box.

Japan almost conceded in the 20th minute after playmaker Salaah Al-Yahyaei caught their defense off guard with a through-ball to striker Muhsen Al Ghassani, who beat keeper Shuichi Gonda with a close-range shot that skidded just wide.

Having ensured their berth in the round of 16 by winning the first two of their three Group F matches, the Samurai Blue will face their final group opponents, Uzbekistan, at Al Ain's Khalifa bin Zayed Stadium on Thursday.